Accurate measurement of surface shape is important for a number of structural applications. For example, spacecraft with optical payloads may need to know the orientation of optical payload supporting structure to a very high degree of accuracy. By measuring any bending deformation or other changes in this supporting structure, the orientation of the optical payload may be determined. In turn, the optical payload may be adjusted responsive to the measurement of the supporting structure to maintain precision pointing of its optical line-of-sight (LOS). Alternatively, a spacecraft with a large phased-array payload may need to determine the surface deformation of the array-supporting structure with very high precision so that the phasing of the phased-array antenna may be adjusted accordingly to maintain a desired beam direction. Similarly, many communication satellites also have a need to precisely measure their antenna-supporting surfaces to meet their antenna-pointing budgets and thereby provide adequate communication service. Accurate structural measurement is important in all “smart structure” systems that utilize deformation information to enhance functionality.
A conventional approach to determine surface deformation in structures has been to distribute strain gauges across the structure to measure structural strains. However, both the structural strains and the strain sensor response typically varies with temperature. Thus, it is difficult to achieve high-precision measurement of surface deformation using a network of distributed strain sensors across a range of operating temperatures. Given the same surface deformation but a range of temperatures, a strain-sensor approach to surface deformation measurement may generate different measurements.
To provide greater measurement precision, laser metrology systems have been used in a number of industrial applications. These systems generally align the lasesr beam axis with the direction of the deflection to be measured. For surfaces, the laser source is displaced away from the surface such that the laser beam propagation is generally normally-directed or close-to-normally-directed with respect to the surface. For example, if the surface is considered to lie in the x-y plane, the laser beam propagation would be in the approximately in the z direction. Sensors on the surface would then measure the distance from the laser source with respect to the propagation direction. Aside from noise considerations due to decreasing signal strength, it may be shown that for larger surfaces illuminated by a single source, the precision and uniformity of such a measurement system increases as the laser source is further displaced from the surface being characterized and surface orthogonality to all points on the surface is approached.
Although the traditional method of laser metrology with respect to characterizing surface deformation may be used in a number of industrial applications, it is relatively impractical to implement this method in a space-based application. For example, a space structure having a dimension of 15 meters would require the laser source to be deployed a comparable distance away from the antenna array surface. Given the constraints faced during launch, the laser source would likely be supported by a deployable structure rather by a stable reference point on the spacecraft. Vibration and thermal deformation of the resulting deployed laser source structure would thus make accurate characterization of the surface deformation much more difficult and complex. Moreover, spacecraft surfaces oriented close to the ecliptic plane (typical for earth-facing surfaces on geostationary spacecraft) can be subject to direct sunlight exposure. Normally-oriented optical sensors can be saturated for long periods or damaged. This problem is particularly acute for any sensor with a wide field-of-view. Although narrowband filters may be used at the sensors to filter out the sunlight, the residual solar energy may still degrade the performance of the traditional laser metrology system.
Accordingly, there is a need in the art for improved laser metrology systems to measure the deformation of structures.